Detect two objects colliding, and write the frame number when collision happened
You can use Animation Nodes for this. We compute the distance between the two target objects, if the distance is smaller than the sum of the radii of their bounding sphere, we create a text from the current frame and if not create an empty text, we read an initially empty text block and append the result to it with a line break as separator (Though a separator of your choice can be used), then we write the output to the text block again. Now as soon as they collide, the frame will be written in a new line.